May is turning out to be so warm in Sydney that authorities are keeping outdoor pools open longer than usual and at least one council is reviewing lifeguard numbers to cope with late-season beach bums.

Sydney, along with much of south-eastern Australia, is in the midst of an autumn heatwave, with daily maximums five degrees or more above average – with most of next week promising the same.

“For Sydney and Canberra, this was the warmest week this late in the season on record,” said Ben Domensino, a senior meteorologist with Weatherzone.

Adelaide's top of 27.4 degrees on Friday was that city's warmest on record this late in the season, while Hobart's 23.9 degrees on Thursday was that city's warmest so deep into autumn. Both cities' records go back more than a century, said Blair Trewin, a senior climatologist at the Bureau of Meteorology.

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“It's been significantly warmer than average throughout the country” with only a few cool pockets on the Queensland and WA coast, Dr Blair Trewin said.

For Sydneysiders, conditions appear perfect for many outdoor activities. The forecast is for sunny days stretching out towards next weekend, with days of 24-25 degrees forecast until at least next Friday - well above May's long-run average of 19.4.

No need to put away the beach towel just yet. Photo: Steven Siewert

Prior to today's top of 25.3 degrees, Sydney had only had 21 days of 25 degrees or warmer weather in the second half of May in more than 150 years of records, said Rob Sharpe, a Weatherzone meteorologist. That tally will swell by at least another six to 27 before May is out, according to the bureau's forecasts.

Swim still in

For lifeguard services and pool operators, though, the unseasonal warmth has prompted staffing headaches as councils weigh up adding to lifeguard numbers. Volunteer lifesavers typically monitor beaches only until the end of April.

Jennifer Mann and her dogs Bella and Khan walk on Stanwell Park Beach, south of Sydney. Photo: Kirk Gilmour

“This weekend's going to great, and next week's going to be great, so we'll keep an eye on it,” said Reece Heddle, manager of aquatic services for Randwick City Council and the Des Renford Aquatic Centre. “If we get more crowds or dangerous conditions, we'll call more (lifeguards) in.”

Mr Heddle said the mild conditions mean the heated outdoor pool he runs will stay open for a while yet – unusual for this late in the year.

“We're not looking at closing it in a hurry,” he said. “It's purely because the overnight temperatures have been so high...I'm not losing so much heat.”

The pool cools a lot when temperatures dip below 12 degrees but locally most nights have seen minimums of 14-16 degrees, he said.

Ocean temperatures off Sydney remain about 22 degrees, or 2-3 degrees above average for this time of year, according to Integrated Marine Observing System data.

Record runs

Last year was Australia's record warmest in more than 100 years of reliable data and meteorologists says this year won't be far behind.

May should see a slew of temperature records fall, particularly in the country's south-east.

Sydney should notch its seventh straight day above 22 degrees on Friday and match the record of nine for any time in May – set in 1978 and 2007 – by Sunday, said the bureau's Dr Trewin.

The city has only had three days of 24 degrees in a row twice this late in the year – in 1923 and 1975 – and it may get eight of them on current forecasts, he said.

Melbourne is also in the midst of a warm spell. The city is forecast to have at least 10 days of 20-degree days up to next Tuesday, easily eclipsing the previous longest warm spell this late in the year of seven days, Weatherzone's Mr Sharpe said.

The immediate cause of the balmy conditions is the blocking high-pressure systems in the Tasman, which are pushing most cold fronts further south than usual – bad news for farmers hoping for rains.

“At this time of year, we get our rain from the passage of the frontal systems,” said Weatherzone's Mr Domensino. “The highs are keeping the skies clear, which is allowing those temperatures to creep up, but it's also preventing any rainfall.”